Liquid crystal color displays which cool from isotropic to twisted nematic to smectic

ABSTRACT

A color display formed of information of one color on a different color background is projected onto a screen. Light from the bulb is passed serially through a neutral polarizer, a smectic liquid crystal cell, and a further polarizer. The cell comprises a smectic liquid crystal material incorporating an amount of a cholesteric material and a dye contained between two cell walls carrying electrodes and surface treated to give a molecular twist and alignment to the liquid crystal material. Information is written into the cell by scanning the laser beam over the cell, causing localized heating, in combination with or without an applied electric field to form selected areas with a twisted structure on a background of homeotropic structure, or vice versa. Thus plane polarized light is passed through the cell with its plane selectively rotated or non rotated. This provides a two color display of information of one color on a background of the other color. Alternatively the two colored polarizers may be replaced by a single neutral or colored polarizer to give a black and white or colored and white display. The display system may be replicated using differently colored polarizers and the resulting images combined into a multi color image on a single screen.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to liquid crystal colour displays in whichinformation is displayed in one colour on a background of anothercolour.

2. Discussion of Prior Art

One known form of colour display uses a smectic liquid crystal materialaddressed by a laser in projection displays and digital storage devices;see for example F. J. Kahn, Applied Physics Letters, 22, p. 111 (1973);A. G. Dewey, Optical Engineering, Vol. 23, No. 3, p. 230 (1984); and H.Birecki et al, SPIE Proceedings Vol. 420 edited by A. E. Bell and A. A.Jamberdino (1983).

A typical projection display with smectic liquid crystal materials usesa scattering and a transmissive state of the smectic material to providea display. Both the light scattering and transmissive states can bestable and co-exist in different parts of a display at the same time.Information is written as areas of light scattering texture on abackground of light transmissive texture i.e. dark features on a lightbackground; or visa versa. The display is monochrome. Multi-colour maybe provided by using a plurality of systems, each with an associatedfilter, and combining the outputs say on a screen. This is quitecomplicated and expensive. The displays need a small highly localisedlight source, e.g. an arc lamp, and Schlieren stop optics. These arewasteful of power and make it difficult to obtain adequate displaybrightness.

SUMMARY OF THE INVENTION

According to this invention these difficulties are overcome by providinga two colour display using a single cell of smectic material arranged asa polarisation switch together with coloured polarisers. The two coloursmay be black, considered as a colour or absence of light, and white,considered as a colour or the absence of other colours; or two colourssuch as red and green.

According to this invention a liquid crystal colour display comprises asmectic liquid crystal cell formed of a layer of smectic liquid crystalmaterial located between two cell walls both coated with sheetelectrodes for applying an electric field across the smectic materiallayer, a light source for illuminating the cell, a lens system fordirecting light from the light source via the cell to a display screen,a laser light source for locally heating the cell, and a scanning systemfor scanning laser light across the cell, characterised by:

a cell in which the smectic material has a nematic phase between thesmectic and isotropic phase,

a homogeneous surface alignment treatment to the facing surfaces of thecell walls, the alignment direction on one wall being at least 45° tothat on the other wall,

a neutral plane polariser between the light source and cell,

at least one polariser positioned to receive light from the cell,

the arrangement being such that liquid crystal material cooling rapidlyfrom the isotropic state in the absence of an applied electric fieldadopts a twisted light rotating configuration, and in cooling from theisotropic state in the presence of an electric field adopts anon-rotating configuration

whereby the two polarisers and the two different liquid crystalconfigurations cooperate to display information in one colour on thebackground of the other colour.

The polariser receiving light from the cell may be a neutral greypolariser, in which case a black and white display is produced; anon-black coloured polariser alone, in which case a non-black colour andwhite display is produced; or two different non-black colouredpolarisers may be used to produce a display of two different colourswith information of one colour on the background of the other colour.

The two coloured polariser are preferably arranged with their optic axisorthogonal to one another and orthogonal or parallel to the alignmentdirection on the nearest cell wall.

A small amount of cholesteric material, e.g. 0.1 to 10% wt., may bedissolved in the smectic material to assist in providing a uniform twistdirection.

The alignment direction on the cell walls may be 90°+/-10° preferably+/-5° or less.

A small amount of a dye, absorbing to the laser wavelength, may bedissolved in the liquid crystal material.

The light source is preferrably an incandescent filament bulb such as ahalogen bulb, backed by a reflector.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, withreference to the accompanying drawings of which:

FIG. 1 is a schematic layout of a prior art system providing amonochrome display;

FIG. 2 is a sectional view of the cell used in FIG. 1;

FIG. 3 is a phase diagram for material in the system of FIG. 1;

FIG. 4 is a schematic layout of a two colour display according to thepresent invention,

FIG. 5 is a view of the cell used in FIG. 4;

FIG. 6 is a sectional view of the cell in FIG. 5; and

FIG. 7 is a phase diagram for material in the system of FIG. 4.

DETAILED DISCUSSION OF-4-PREFERRED EMBODIMENTS

The prior art projection system shown in FIG. 1 comprises in serialorder a mercury arc lamp 1, a condensor lens 2, UV and infra red filters3, 4, a dichroic beam combiner 5, liquid crystal cell 6, projector lens7, a Schlieren stop 8, and a forward scattering screen 9. Additionally aHe-Ne laser 10 has its light beam 11 scanned in both an x and a ydirection by a two mirror system, shown in outline only at 12, via ascan lens 13 onto the dichroic beam combiner 5 and thence on the cell 6.This system is usefully combined with a second projection system 14,such as a map display, whose output is focussed onto the screen 9. Themap display comprises a light source 15, two lenses 16, and 17, togetherwith a slide holder 18 carrying a slide of a map.

The mercury arc lamp 1 is a 350 W lamp giving as close to a point sourceas possible so that the cell 6 is illuminated with a collimated beam oflight. The cell 6 is highly scattering so that it is neccessary toremoved unwanted scattered light by the Schlieren stop 8. This requiresa small point source of projector light which makes the systeminefficient. Thus for viewing in conditions of high ambient light it isdifficult to obtain an adequately bright display.

The cell FIG. 2, comprises a 12 μm thick layer of S2 (BDH material)located between two glass walls 21, 22. A small amount of dye isdisolved in the material to absorb the laser radiation and hence producelocalised heating in the layer 20. The inner surface of each wall iscoated with a transparent electrically conducting coating of indium tinoxide 23, 24. These are used to apply an electric field across the layer20, and may also be used as resistance heaters to maintain the layertemperature at a desired value. Both walls 21, 22 are treated to give ahomeotropic alignment to contacting liquid crystal molecules, thisimparts a tendency for the contacting molecules to order themselvesperpendicular to the walls.

The dichroic beam combiner 5 is reflecting to the HeNe laser wavelengthand also to red wavelengths from the lamp 1; the latter is to avoidunwanted heating of the cell. Unfortunately there still exists anundesired temperature gradient across the cell due to the non uniformlight output from the arc lamp 1. The combiner 5 is transparent to allother visible wavelengths from the lamp.

Typically the laser is of 8 mW output which gives a writing speed ofabout 1 m/s on the screen.

Information is displayed on the screen 9 by causing areas of the layer20 to become light scattering on a transparent background, or viceversa. This is explained with reference to FIG. 3.

It is a feature of smectic A type liquid crystal materials such as S2,that they can exist indefinitely in either of two states or textures;

(a) a clear state in which the optic axis is aligned uniformlyperpendicular to the walls; and

(b) a dense scattering, spherulitic focal conic state, in which thespatial extent of aligned regions is small typically about 1 μm.

For S2 material a phase transition between smectic and nematic phasesoccurs at 48° C., and a further phase change between nematic andisotropic occurs at 49° C.

Application of a strong A.C. electric field, e.g. above 5×10⁶ Vm⁻¹ willchange the texture from scattering to clear in the smectic phase.Localised heating of the layer 20 onto the isotropic phase is achievedby absorption of laser radiation in the layer 20. This isotropic phaseis a disordered structure and is the result of heating either thescattering or clear smectic textures above the 49° C. transitiontemperature.

From the isotropic phase two routes are possible. Rapid natural coolingfrom the isotropic through the nematic to the smectic phase results inthe scattering texture (b). Rapid natural cooling in the presence of amoderate electrical field of about 5×10⁵ Vm⁻¹ results in a clear smectictexture (a). Thus areas of scattering texture and areas of clear textureare obtained by localised heating with the laser 10 and cooling withselective application of an applied electric field. This is used toselectively write and erase information onto the screen. Erasure of thewhole cell 6 is obtained by application of the high field to the layer20.

The display on the screen is monochromatic, i.e. light and dark featuresin a single colour. Multicolour can be achieved by using a plurality ofthe systems of FIG. 1 in combination with different colour filtersarranged between the cell 66 and Schlieren stop 8. This is costly andneeds accurate alignment of all systems to provide a multicoloureddisplay.

A two colour system of this invention is shown in FIGS. 4 to 7. FIG. 4shows a layout similar to that of FIG. 1. However, the lamp 30 is aconventional incandescent filament projector bulb and reflector 30'since it not necessary to use a point source bulb. Also no Schlierenstop is needed. Light from the lamp 30 is directed by a condensor lens31 through UV and IR filters 32, 33 and a neutral polariser 34 arrangedwith its polarisation axis vertical. After the polariser 34 light passesthrough a dichroic beam combiner 35, a liquid crystal cell 36, a firstcolour polariser 37, a second colour polariser 38, and projection lens39 onto a screen 40. The first colour polariser 37 is arranged with itspolarisation axis vertical whilst that of the second colour polariser 38is horizontal. The coloured polarisers may be red and green polarisersrespectively. An advantage of the lamp 30 is that it provides areasonably uniform temperature gradient across the liquid crystal cell36. This makes laser addressing easier because the cell can be operatednearer to the nematic phase transition and lower power lasers used foraddressing.

As in FIG. 1 a He-Ne laser 41 has its output beam 42 directed via a twomirror system 43, scan lens 44, and the dichroic combiner 35 onto thecell 36.

This two mirror system 42 comprises two mirrors 45, 46 rotatable aboutorthogonal axes 47, 48 by motors 47' and 48' to cause a horizontal andvertical scanning of the laser beam 42 via a fixed mirror 49 onto thecell 36.

The cell 36, FIGS. 5, 6, comprises two glass walls 51, 52, 1.5 mm thickand spaced 12 um apart by a spacer ring 53. The inner surface of bothwalls are coated with a 1000 Å thick layer of transparent electricallyconducting indium tin oxide 54, 55. Prior to assembly the walls aresurface treated to provide a homogeneous alignment to liquid crystalmolecules in a conventional manner. This alignment may be by obliqueevaporation of SiO or coating with a polyimide such as Nolimide 32(T.M.) and unidirectionally rubbed with a rayon cloth. Such treatmentcauses alignment of the liquid crystal molecules in a single directionalong the rubbing direction. Preferrably one wall is treated by obliqueevaporation at 30° angle to give alignment with a zero tilt tocontacting liquid crystal molecules; and the other wall is treated byoblique evaporation at about 5° to give alignment with a molecular tiltof about 30°. The walls 51, 52 are assembled so that the alignmentdirections are orthogonal as indicated by arrows R1, R2. The cell 36 isarranged so that direction R1 on the wall nearest the neutral polariser34 is horizontal i.e. orthogonal to the optical axis of the neutralpolariser 34.

A smectic A liquid crystal material 56 is contained between the walls51, 52. This material 56 may be S2 BDH material 4-n-alkyl and4-n-alkyloxy 4'-cyanobiphenyl together with about 1% of C15 BDHcholesteric material (-)4-(2Methylbutyloxy) 4'-cyanobiphenyl. Othersuitable materials are BDH materials catalogue numbers S1, and S3 to S6.A small amount e.g. up to 5%, typically 2% of a dye is added to absorbthe laser radiation and cause localised heating in the smectic layer 56.Suitable dyes are as described in an article by F. C. Saunders et alIEEE Transactions on Electron Devices, Vol. E-D30, No. 5 p. 499 (1983).A dye suitable for use with a laser operating in the near infra-red isdescribed in (U.K. P.A. 8323359), e.g. a nickel dithiene derivative dyesuch as the ICI dye number SC100870.

A voltage source 57 is arranged to supply voltages to the electrodes 54,55. Control of the scan mirrors 45, 46, and laser 41, and voltages fromthe voltage source 57 is from a control unit 58. This may containstandard routines for scanning conventional symbols onto the cell 36.

As shown in FIG. 7 a cell filled with S2 and C15 has a smecticnematicphase transition at 48° C., and a nematic-isotropic phase transition at49° C. On cooling from the isotropic phase in the absence of an appliedelectric field this material adopts a twisted nematic state in thenematic phase. This twist is induced by the orthogonal surfacealignments R1, R2. The cholesteric material C15 ensures a uniform twistdirection. On further cooling from the nematic to smectic phase withoutan applied electric field, the material retains its twisted structure inthe smectic phase. Such a twisted structure is optically active; itrotates the plane of incident plane polarised light by 90° and issimilar to the known twisted nematic effect.

Cooling the material from the isotropic, through nematic, to the smecticphase in the presence of an applied electric field of about 5×10⁵ Vm⁻¹causes the material to adopt the clear texture with the moleculesaligning perpendicular to the walls 51, 52.

Both the twisted structure and clear structure can be locally heatedinto the isotropic state by laser heating. The twisted texture can bechanged into the clear texture by application of a strong electric fielde.g. above 5×10⁶ Vm⁻¹.

In operation for example a red colour is obtained on the screen 40 whenthe cell 36 is in its clear texture. Light from the lamp 30 isvertically polarised by the neutral polariser 34, passes unchangedthrough the cell 36 to be filtered by the red polariser 37 so that onlyred light passes to the screen 40. When the cell 36 is in its rotatedtexture vertically polarised light from the neutral polariser 34 isrotated by the cell 36 to emerge horizontally polarised. Thishorizontally polarised light is filtered by the green polariser 38 toilluminate the screen 40 with green light.

Thus the laser beam 42 is scanned over the cell 36 without any appliedvoltage so that areas of smectic material adopt a twisted texture in aclear background. This gives green areas on a red background, where thegreen areas are shaped to give information such as numbers, letters, orsymbols. These green areas may be selectively erased by scanning thelaser beam 42 over them and applying a moderate electric field as thelocally heated material cools into the clear texture.

Alternatively the display can be red on a green background. In this casethe whole cell 36 is changed into the twisted texture and the laser beam42 scanned over the cell 36 in the presence of a moderate electricfield. Areas of clear texture are thus formed on a background of twistedtexture in the layer 56. The colours may also be reversed by reversingthe relative position of the red and green coloured polarisers 37, 38 orby rotating their optical axes by 90°.

Other colours and combinations are obtained by use of different colourpolarisers.

More than two colours may be obtained by replacing the two colourpolarisers with a multi colour switch as described in GB 1,469,638.

For example the two colours black and white may be obtained by replacingthe two coloured polarisers 37, 38 with a single neutral polariseraligned parallel or orthogonal to the plane of light emerging from thecell 36. Alternatively a coloured, e.g. red, and white display may beobtained using e.g. a red polariser. A coloured e.g. blue and black e.g.zero light, may be obtained using a blue polariser and a neutralpolariser arranged with their optical axis orthogonal to one another asin FIG. 4.

Thus a three colour, red, green, and blue display may be obtained usingtwo cells 36 side by side along different optical paths and combiningthe two paths onto a screen. One such cell may provide the red and greencolour, and the other cell blue and black (absence of light).Alternatively three colours may be obtained using three cells arrangedalong three different optical paths. Each cell has associated therewithtwo polarisers, one a neutral polariser to give black (no light) and acoloured polariser. The three optical paths are combined to give asingle image on the screen 40. For example the arrangement of FIG. 4 canbe replicated three times, with different colour polarisers, theresulting three images combined on the screen 40 into a singlemulticolour image. Such three colour displays are more efficient thanthe prior art because the light source is a conventional, incandescentfilament, bulb and no Schlieren stop is needed.

We claim:
 1. A liquid crystal colour display comprising:a smectic liquidcrystal cell, said cell comprising:two cell walls, each of said wallscoated with a sheet electrode for applying an electric field across thecell; a smectic liquid crystal material layer located between saidwalls, said liquid crystal material having a nematic phase betweensmectic and isotropic phases; and a homogeneous surface alignmenttreatment on said cell walls, the alignment direction on one wall beingat least 45° to the alignment direction on the other wall, a lightsource for illuminating the cell, a lens system for directing light fromthe light source via the cell to a display screen, a laser light sourcefor locally heating at least a portion of said liquid crystal materialto an isotropic phase a scanning system for scanning laser light acrossthe cell, a neutral plane polariser between said light source and saidcell, and at least one polariser positioned to receive light from thecell, said laser light source, said surface alignment treatment, andsaid cell walls comprising a means for cooling said liquid crystalmaterial rapidly from an isotropic state and for causing the liquidcrystal material to adopt a twisted light rotating configuration in theabsence of an applied electric field, and for causing the liquid crystalmaterial to adopt a non-rotating configuration in the presence of anelectric field.
 2. The display of claim 1 wherein the alignmentdirections on the cell walls are 90° plus or minus 10° to one another.3. The display of claim 1 wherein up to 10% of a cholesteric material isincorporated in the smectic liquid crystal material.
 4. The display ofclaim 1 wherein the polariser positioned to receive light from the cellis a single neutral polariser.
 5. The display of claim 1 wherein thepolariser positioned to receive light from the cell is a single colouredpolariser.
 6. The display of claim 1 wherein the polariser positioned toreceive light form the cell is formed by a neutral polariser and acoloured polariser.
 7. The display of claim 1 wherein the polariserpositioned to receive light from the cell is formed by two differentcolour polarisers.
 8. The display of claim 1 wherein the light source isan incandescent filament bulb.
 9. The display of claim 1 wherein anamount of an infra red radiation absorbing dye is incorporated in theliquid crystal material.
 10. A liquid crystal color displaycomprising:(a) a plurality of smectic liquid crystal cells, oneassociated with each color to be displayed, each of said cellscomprising:two cell walls, each of said walls coated with a sheetelectrode for applying an electric field across the cell; a smecticliquid crystal material layer located between said walls, said liquidcrystal material having a nematic phase between smectic and isotropicphases; and a homogeneous surface alignment treatment on said cellwalls, the alignment direction on one wall being at least 45° to thealignment direction on the other wall; (b) at least one light source forilluminating said cells; (c) at least one lens system for directinglight from the light source via said cells to a display screen; (d) atleast one laser light source for locally heating at least a portion ofsaid liquid crystal material to an isotropic phase in at least one ofsaid cells; (e) at least one scanning system for scanning laser lightacross said cells; (f) a plurality of neutral plane polarisers, oneassociated with each of said cells and located between said light sourceand said cell; and (g) a plurality of polarisers, at least one polariserassociated with each cell and positioned to receive light from the cell,wherein said laser light source, said surface alignment treatment, andsaid cell walls associated with each of said cells comprising a means,associated with each cell, for cooling said liquid crystal materialrapidly from an isotropic state and for causing the liquid crystalmaterial to adopt a twisted light rotating configuration in the absenceof an applied electric field, and for causing the liquid crystalmaterial to adopt a non-rotating configuration in the presence of anelectric field.